Cardiac Imaging: New Modalities

MONTREAL, Sept. 15 -- With an impressive array of new imaging technologies hitting the market, nuclear cardiologists need to watch utilization carefully.

The key, said Dr. Rory Hachamovitch, M.D., of the University of Southern California, is to understand the benefits and limitations of each new modality.

"Over a fairly short period of time, cardiologists have been hit with several new modalities," he said at an industry symposium, sponsored by Bristol-Myers Squibb Medical Imaging, held in conjunction with the American Society of Nuclear Cardiology here. But he called for restraint.

"If we start doing CT angiography (CTA) on everyone, the healthcare system in the United States will go broke," he noted. "Also, if you think about the radiation dose people are going to get, particularly if you do sequential testing with nuclear, it's very problematic."

One solution is to conduct research aimed at determining exactly what type of information each modality offers, how reliable it is, and which situations it is most likely to offer clinically relevant information. The next step is to educate physicians about the appropriate use of these new tools, said Dr. Hachamovitch and his colleague, Michael Y. Shen, M.D., of the Cleveland Clinic, Florida.

Coronary calcium score, as determined by CTA, is gaining popularity as a method of quantifying atherosclerosis, which in turn offers predictive value for coronary artery disease risk independent of other traditional risk factors, they said.

But the role of CTA remains equivocal, said Dr. Hachamovitch. In two recent studies examining patients who underwent both stress single photon emission computed tomography (SPECT) and CTA revealed that while CTA has an outstanding negative predictive value for the presence of ischemia, often over 95%, its positive predictive value is poor, only about 30 to 40%.

Another recent study demonstrated that while CTA is excellent at determining the presence of atherosclerosis, it cannot reliably determine the degree of stenosis present. "We have yet to see compelling evidence that this is a replacement for an angiogram," said Dr. Hachamovitch.

To help clinicians develop a clear idea of situations in which to use new modalities such as myocardial perfusion imaging (MPI) and CTA, as well as what types of data they can expect to obtain from these imaging studies, Drs. Hachamovitch and Shen presented four cases in which these tests were considered and discussed why they were used and what they revealed.

The first case was that of a 36-year-old asymptomatic male with a history of hypertension and hypercholesterolemia, both being controlled with drug therapy. His father had undergone coronary bypass grafting in his 30s. The patient was taking aspirin 81 mg/day, Cozaar (losartan) at 50 mg, and Zocor (simvastatin)at 20 mg/day.

The investigators performed CTA on this patient to determine the presence of calcified and soft plaque, and the study demonstrated multiple areas of atherosclerosis in both the right coronary artery and left anterior descending artery. To determine the presence of functional ischemia, they performed a SPECT MPI, which yielded a normal result. The clinical recommendation was to double the Cozaar dose and to provide more aggressive statin therapy by switching the patient to a high dose of Lipitor (atorvastatin).

According to Dr. Hachamovitch, the take home message from this case is that CTA can detect significant atherosclerosis even in asymptomatic patients, but SPECT MPI is required to determine the presence of ischemia. "It is completely feasible and actually quite common to see ? what by CTA looks like significant stenosis and have completely normal perfusion," said Dr. Hachamovitch. "It is far more common than people think. ? Assessment of atherosclerotic burden with CTA may help tailor aggressive cholesterol treatment for patient care, although validity quantification of atherosclerosis and clinical outcomes of treatment is not established."

The second case was a 63-year-old male with atypical chest pain. He had no sign of diabetes and was a non-smoker, but he had a history of hypercholesterolemia, for which he was taking medication. Five months previously he had received a stent placement in the mid-left anterior descending artery, and there was no sign of obstructive stenosis.

A CTA study revealed a very high coronary calcium score of 2146. While the stented section was patent, there were signs of what appeared to be moderate disease in the left circumflex coronary artery and less severe disease in the right coronary artery. Next, a SPECT MPI revealed an ejection fraction of 62% and signs of ischemia. This patient underwent catheterization.

This case, said Dr. Hachamovitch demonstrates that CTA can detect the presence of de novo coronary artery disease in a patient who underwent stenting. The SPECT MPI prior to catheterization helped determine the location of significant ischemia. Questions still remain, however. For instance, will the future role of CTA vs. MPI be dependent on the pretest likelihood of restenosis?

Case three was a 46 year-old male with chest pain. The patient was obese, had hypercholesterolemia and a family history of coronary artery disease. He was also a smoker and heavy drinker. This patient was immediately sent for a SPECT MPI, which received an equivocal interpretation from a radiologist but was considered a normal result by a cardiologist. Because of these uncertain findings, the patient underwent CTA, which revealed mild to moderate plaque formation in the left anterior descending and right coronary artery but no sign of stenosis. Aggressive statin therapy was recommended, but the patient soon returned with severe chest pain radiating down his left arm. This time, he underwent catheterization, which revealed normal coronary arteries and preserved systolic function.

This case demonstrates, said Dr. Hachamovitch, that when MPI SPECT is equivocal, CTA can help in defining the presence of coronary stenosis. It can have a high negative predictive value, if the quality of the study is adequate.

The final case was a 76 year-old asymptomatic male with no prior history of coronary artery disease who presented for a routine check up. He had underdone catheterization 18 years previously. His last SPECT MPI was 14 months prior, and revealed no signs of ischemia. For a case such as this one, said Dr. Hachamovitch, there is no data to guide practice. How closely a patient such as this one should be followed up using medical imaging remains unclear.

Nonetheless, this patient underwent another SPECT MPI, which demonstrated a small amount of ischemia that had not been present on the previous test. There was also a small dip in ejection fraction, from 73 to 70% pre and post stress, which could be an element of transient ischemic dilation. A subsequent CTA demonstrated the presence of significant disease, which was later confirmed by catheterization.

The take home message from this case, said Dr. Hachamovitch, is that CTA can help identify underestimations of disease by MPI when ancillary markers are present but the diagnosis is not clear-cut. CTA may also assist in the planning of complex revascularization procedures before the patient undergoes catheterization, particularly in patients who have undergone prior CABG.

"I hate piling up tests; I think it gets excessive," said Dr. Hachamovitch. "But these tests aren't perfect, and there are many types of nuclear results that are problematic, where you don't really know what to do with the patient. ? Rather than sending that patient to the cath lab, you can do a CTA. That's only going to be 5%, 10%, maybe 15% of patients. ? The hope is that it acts as a filter and the majority of cases won't need [catheterization]. That's where you're going to get the cost savings."

To help further elucidate what information these newly available modalities offer and when they should be used, Dr. Hachamovitch and colleagues are recruiting for the Study of perfusion versus anatomy's role in coronary artery disease (SPARC) trial. The 4,000 patients in this trial will undergo SPECT, positron emission tomography (PET), PET/CT, and or CTA in an effort to compare these modalities in various clinical situations and to determine cost effectiveness.

• Arad Y et al. J am Coll Card. 2005;146:158-165
• Hacker M et al. J Nuc Med. 2005;46: 1294-1300
• Schuijf et al. J Amer Cardiol. 2006 (in press)
• Leber AW. J Am Coll Cardiol. 2005;46:147-54.